The operator of a cellular telephone system often has to provide telephone service to profitable and more densely populated city areas as well as relatively less profitable and less densely populated rural areas or coastal areas. It is therefore desirable that a cellular telephone system cover as much area as possible with a minimum of equipment. One method of accomplishing this objective is to use relatively large cells.
The first cellular mobile radio systems placed in public use were analog systems that were typically used for speech or other types of analog information. These systems include multiple radio channels for transmitting analog information between base stations and mobile stations by transmitting analog modulated radio signals. One such system is the Nordic Mobile Telephone System NMT-450. Another known cellular radio system is the AMPS Mobile Radio System in the United States. The rapidly increasing usage of mobile radio systems has necessitated development of newer, more advanced digital systems that can accommodate a larger number of mobile stations using time division multiple access (TDMA) technology or code division multiple access (CDMA) technology.
In digital mobile telephone systems, e.g., GSM, ADC, and JDC, each radio carrier is divided into frames and each frame is subdivided into a number of time slots. Each time slot typically carries one connection (channel), and a burst of data is transmitted during each time slot. These types of systems are referred to as TDMA systems. Due to the finite propagation speed of radio signals, mobile stations at a distance from the base station must transmit a burst of data at a certain time in advance relative to the frame structure perceived in the mobile station in order that the burst of data or radio signal arrives at the base station in the correct time slot.
The base station typically measures the time of arrival for the access burst of data. An appropriate time interval for advanced transmission is calculated and transmitted to the mobile station. In the GSM system, the coding of this time interval, combined with the specified TDMA structure gives a maximum cell radius of approximately 35 kilometers.
In a TDMA system such as the GSM system, mobile stations can share a single carrier signal because the signal is divided into frames. Each frame is subdivided into time slots and mobile stations are assigned to one or more time slots depending upon the transmission mode. Each mobile station transmits bursts of data during its time slot which is advanced as necessary. Various published standards, such as those for the GSM digital mobile system in Europe and the EIA Interim Standard (IS-54) in the United States, set forth the specification for the transmission of a carrier signal that is modulated with the burst of digital data. For example, under the GSM standard, the carrier signal is divided into a frame consisting of eight equal time slots as illustrated in FIG. 1. The carrier signal is called a downlink (DL) when it is transmitted from the base station to the mobile station, and the carrier signal is called an uplink (UL) when it is transmitted from the mobile station to the base station.
In the normal transmission mode as illustrated in FIG. 1, each channel utilizes only one time slot during each uplink and downlink frame. Thus, a single frame can accommodate eight traffic channels under the GSM standard. When operating in the normal transmission mode, the mobile stations must operate at a relatively close distance to the base station, and the effective cell size is limited.
In the conventional extended transmission mode as illustrated in FIG. 2, the effective size of a cell in the GSM system can be enlarged, because the conventional extended range channels have longer time slots which can accommodate longer propagation delays. In the conventional extended transmission mode, the channel on which the data is transmitted utilizes two adjacent time slots in a frame. Thus, under the GSM standard, a single frame can accommodate four conventional extended channels, such as the four uplink channels 0, 2, 4, and 6 of FIG. 2.
It should be noted that the uplink transmission as seen in the base station in both FIGS. 1 and 2 differs from the downlink transmission as seen in the base station by three time slots which is typical for prior art TDMA systems according to the GSM standard. The difference has nothing to do with extended range. In other words, the beginning of the 0 time slot for downlink transmission (base station to mobile station) is advanced exactly three time slots from the 0 time slot of the uplink transmission as seen in the base station (mobile station to base station). The time slots of the base station are considered to be reference time slots.
It is possible to combine normal range channels of FIG. 1 with conventional extended range channels of FIG. 2 in the same cell and even on the same carrier signal. Calls are then set up on a channel of an appropriate channel type, depending upon the distance, and a call can be handed over to the other channel type if the distance changes. The only limitation is that the conventional extended channel must be able to accept handover access bursts within a slot consisting of two normal time slots.
While the conventional extended transmission mode of the GSM system can substantially increase the operating distance of a mobile station from a base station, there is a corresponding decrease in traffic load, because the number of channels is decreased from eight to four. Accordingly, there is a need for a cellular telephone system which can allow mobile stations to operate at a substantially increased distance from the base station without a corresponding decrease in the number of channels.